Comparison of epoxy and braze-welded attachment methods for FBG strain gauges

This paper presents experimental results from fatigue and static loading tests performed on both epoxy and braze-welded FBG strain sensors. Most FBG attachment methods are relatively understudied, with epoxy the most commonly used. Long curing times and humidity sensitivity during curing render epoxy inappropriate for certain implementations. This work shows that a bespoke braze-welded attachment design is able to achieve a higher static failure limit of 22kN when compared to strain gauge epoxies, which fail at 20kN. Both methods demonstrate high fatigue life, with no significant deterioration after two million cycles. Epoxy swelling was observed when the sensors were held at a relative humidity of 96%, applying ~0.6 mϵ of tension to the FBG, whereas a braze-weld attachment was unaffected by humidity

Wind turbine lifetime extension decision-making based on structural health monitoring

In this work, structural health monitoring data is applied to underpin a long-term wind farm lifetime extension strategy. Based on the outcome of the technical analysis, the case for an extended lifetime of 15 years is argued. Having established the lifetime extension strategy, the single wind turbine investigated within a wind farm is subjected to a bespoke economic lifetime extension case study. In this case study, the local wind resource is taken into consideration, paired with central, optimistic, and pessimistic operational cost assumptions. Besides a deterministic approach, a stochastic analysis is carried out based on Monte Carlo simulations of selected scenarios. Findings reveal the economic potential to operate profitably in a subsidy-free environment with a P90 levelised cost of energy of £25.02 if no component replacement is required within the nacelle and £42.53 for a complete replacement of blades, generator, and gearbox

Alkali-activated cement sensors for sodium chloride monitoring

Chloride-induced corrosion of reinforced concrete costs the global economy billions of dollars every year. Despite concerted research effort, the non-invasive, continuous monitoring of sodium chloride in concrete structures is still an unsolved problem. Here, we outline a first-time demonstration of a sodium chloride sensor based on alkali-activated cements: a class of cementitious, electrolytically conductive materials which are typically used for concrete construction and repair. In this work, internal sodium chloride concentrations ranging from 0–22 wt% were measured independently of moisture contents via shifts in electrical impedance. The typical sodium chloride measurement precision was 0.85 wt%. Non-linearity of the sensor response due to signal saturation began at sodium chloride concentrations >5 wt%. We use experimental measures of ion dynamics to link this saturation to the deleterious effect of high concentrations on ion mobility. This study demonstrates sensor feasibility, and provides new experimental evidence to further our understanding of ionic conductivity mechanisms in these materials. The work will allow for the development of self-sensing repair and construction materials for locating and quantifying sodium chloride levels within concrete structures — a valuable technology for supporting concrete health monitoring and maintenance

Predicting freeze-thaw damage using tipping point analysis of strain data

This paper demonstrates how Tipping Point Analysis can be used to predict the onset of structural strain, induced by ice formation. In civil structures, water-ice transitions present many potential issues that can lead to structural damage or plant shutdowns. Examples include freeze-thaw damage in concrete and masonry, and ice build-up on moving parts such as wind turbine blades. Early indication of ice formation could prevent irreparable damage if this information could be used to actuate de-icing procedures. The transition considered in this work is the strain induced in a polypropylene container by the volume change of water as it freezes, measured using surface-mounted fibre-optic strain sensors. This first order phase transition can be detected early on using degenerate fingerprinting, which identifies “slowing down” of the noise prior to the critical point of the transition. Water was supercooled which, at freezing, causes a rapid increase in temperature, presenting an identifiable specific transition point for reference. The analysis was able to consistently predict freezing around 5-10 minutes prior to the transition. A linear relationship was found between mass calculated from the calorimetric equation and mass from experimental measurements. Strain could not be estimated from this mass, since the random process of freezing in an open top container causes an irregular distribution of force. These tests will allow the method and the model to be continually developed towards a more practical application

Ambient Cured Fly Ash Geopolymer Coatings for Concrete

The reinforced concrete structures that support transport, energy and urban networks in developed countries are over half a century old, and are facing widespread deterioration. Geopolymers are an affordable class of materials that have promising applications in concrete structure coating, rehabilitation and sensing, due to their high chloride, sulphate, fire and freeze-thaw resistances and electrolytic conductivity. Work to date has, however, mainly focused on geopolymers that require curing at elevated temperatures, and this limits their ease of use in the field, particularly in cooler climates. Here, we outline a design process for fabricating ambient-cured fly ash geopolymer coatings for concrete substrates. Our technique is distinct from previous work as it requires no additional manufacturing steps or additives, both of which can bear significant costs. Our coatings were tested at varying humidities, and the impacts of mixing and application methods on coating integrity were compared using a combination of calorimetry, x-ray diffraction and image-processing techniques. This work could allow geopolymer coatings to become a more ubiquitous technique for updating ageing concrete infrastructure so that it can meet modern expectations of safety, and shifting requirements due to climate change